1. Field of the Invention
The present invention relates to a nitride semiconductor laser chip and a nitride semiconductor laser device equipped with the nitride semiconductor laser chip. The present invention also relates to a manufacturing method of a nitride semiconductor laser chip.
2. Description of Related Art
A nitride semiconductor containing a group III element and a group V element, which is a so-called group III-V semiconductor (for example, AlN, GaN, InN, AlGaN, InGaN, and the like) is expected to be used as a light-emitting device that emits blue or blue-violet light because of the band structure, and has already been used for a light emitting diode and a laser device.
For example, a nitride semiconductor laser chip has been obtained that has an active layer having a multiple quantum well structure formed by alternately stacking GaN (including the case of containing a trace amount of In) and InGaN layers, and such a chip has been used in various devices because of its stability of light emission (refer to Japanese Patent Application Publication No. 2007-273901)
However, such a nitride semiconductor laser chip has a problem that lattice mismatch occurs between the GaN and InGaN layers. This is attributed to the fact that InGaN is a mixed crystal of GaN (having an a-axis length of 3.16 Å) and InN (having an a-axis length of 3.54 Å), which both have a wurtzite structure but have different lattice constants. Accordingly, when the GaN and InGaN layers are grown in the <0001> axis direction, the active layer suffers from crystal lattice mismatch depending on a composition ratio of In in InGaN. In such a case, stress due to strain introduced to the crystal is increased, and various problems arise, such as a problem of spatial separation of an electron and a hole by a piezoelectric field and a problem of introduction of a large amount of defects. Then, these problems promote a decrease in light-emitting characteristics and an increase in heat generation, which makes the laser device with the nitride semiconductor laser chip prone to break down. This consequently leads to a problem of decreasing the reliability of the laser device. Furthermore, there may also arise the same problem of lattice mismatch not only in the active layer but also in other layer (for example, a layer using AlGaN or the like).